Regulation of root-to-shoot ratio in plant resource allocation by external and internal cues

Plants dynamically adjust their root-to-shoot ratio (R:S) in fluctuating environments, a response often interpreted as adaptive resource allocation, although other explanations have also been proposed. Despite ongoing debates surrounding optimal partitioning theory, the active-regulation framework remains valuable for understanding R:S regulation, which is significant for improving plant resource use efficiency and thus fitness and productivity. This review synthesizes multi-scale advances, from physiology to molecular mechanisms, to outline the regulatory networks associated with coordinated root-shoot growth. We examine how major environmental factors such as light, temperature, osmotic stress, nitrogen, and phosphorus regulate R:S through distinct pathways, highlighting the photoreceptor-mediated carbon-nitrogen allocation, organ-specific thermomorphogenic responses, and the multi-layered osmotic stress networks. For nutrient regulation, we emphasize the OsWRI1a-RNR10-DNR1 module for tissue-specific R:S nitrogen response and the PHR1-centered phosphate starvation response network. We also summarize how endogenous hormones (auxin, cytokinin, abscisic acid, and strigolactone) and signaling hubs like TOR kinase regulate R:S. Finally, we discuss current challenges and future directions for a system-level understanding of R:S regulation and its targeted manipulation.